In these years, increase of memory capacity and improvement in reading accuracy are required in an information recording and reproducing apparatus, and so digital signal processing technology, such as Partial Response Maximum Likelihood (PRML) system, has been introduced in a digital data reproducing apparatus. In regard to phase control in the digital signal processing in which analog signal is converted into digital signal, Japanese Patent Laid-Open No. H09-231506 (corresponding to U.S. Pat. No. 5,760,984) proposes Digital Timing Recovery (DTR) circuit that generates digital data synchronous with the reproduced analog signal by using interpolation from the reproduced digital signal that is sampled asynchronously in an analog-to-digital (A/D) converter.
Here, FIG. 10 shows a configuration example of a conventional digital data reproducing apparatus for reproducing recorded data from signal read from a recording medium. Reproduced analog signal, which is read from a recording medium 100 by read head 101, is sampled in asynchronous with the reproduced analog signal in A/D converter 102, and then converted into reproduced digital signal. Next, the reproduced digital signal is supplied to DTR circuit 104 after digital correction processing in correction circuit 103. Digital data generated in the DTR circuit 104 is supplied to detector 105, and reproduction processing of detected recorded data is performed.
At this time, if off-track state occurs in the reproducing apparatus, amplitude of the reproduced analog signal becomes small and it becomes difficult to maintain phase synchronization. As a result, data reproduction error occurs due to unstable phase control. When the reproducing apparatus returns to on-track state and amplitude of the reproduced analog signal becomes big again, phase control starts, and then phase control becomes stable after a certain period of control delay, and phase synchronization occurs.
Japanese Patent Laid-Open No. H10-64176 discloses a device that is equipped with an off-track detecting circuit for detecting an off-track of a magnetic head and a data buffer memory for storing write data from a host system. In the device, when the off-track detecting circuit detects an off-track during data writing, data is read from the sector that the off-track occurred after the data writing completion, and this read data is compared with the written data stored in the data buffer memory.
Additionally, if there is lack of data in inputted data, data error occurs after the lack portion for a period between a time point when phase synchronization response becomes unstable and a time point when it becomes stable, even if the inputted data itself has no defect. Therefore, when reproducing data error occurs, in order to shorten period of the data error, a method is disclosed in Japanese Patent Laid-Open No. 2000-48490 that processes sampling data in the reverse order that is contrary to the order inputted to phase synchronization signal circuit during the period that phase synchronization response is unstable.
In the above-mentioned conventional digital data reproducing apparatus, however, in regard to the period between a time point when phase control starts and a time point when phase synchronization occurs, data reproduction error occurs due to unstable phase control, though reproduced analog signal with data reproduction capability is inputted. This is because area for phase control is not always prepared when phase control starts at midway point of a track and control gaining is processed utilizing data area, though area for phase control is prepared when data reproduction is performed from the beginning of a track. In these years, track pitch of recording medium is more and more narrowing for actualizing larger recording capacity. In order to record on or reproduce from a recording medium with narrow tracks, extremely precise mechanical accuracy is required, but the realization is becoming difficult. In particular, in a recording and reproducing apparatus with interchangeable media, variation between units has big influence on accuracy. As a result, tracking in reproduction of normally recorded tracking portion becomes unstable, and data reproduction error can occur frequently such as off-track state, or track deflection (or skew) in writing data on a track. In some cases, it cannot be covered by error correction.